Faster groupwise joins to complete implicitly missing rows

I have a grouped DataFrame with dates and values but only observe dates in which a value was observed. I’d like to create the rows that are implicitly missing and have them as missing e.g. for interpolation later. I came up with the following code but was wondering if there is a faster way to do this?

df = DataFrame(
    g = ['a','a', 'b', 'b', 'c', 'c', 'c'], 
    date = [Date(2021,1,1), Date(2021,1,2), Date(2021,1,2), Date(2021,1,4), Date(2021,1,1),Date(2021,1,3) ,Date(2021,1,7)],
    v = rand(7)
alldates = DataFrame(date = minimum(
gdf = groupby(df, :g)
combdf = DataFrame()
for g in gdf
    gout = leftjoin(alldates, g, on = :date)
    gout.g .= g.g[1]
    disallowmissing!(gout, :g)
    append!(combdf, gout, cols = :union)

Not sure if faster, but I think this is clearer:

julia> combdf2 = rename!(DataFrame(Iterators.product(, unique(df.g))), [:date, :g])
julia> combdf2 = leftjoin(combdf2, df, on = [:date, :g])

julia> isequal(combdf, combdf2)

Thanks! It is not only clearer but also twice as fast in a quick benchmark!

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